How to apply opacity to png texture
Since I don't have access to your textures, I can't show you how to hook them all up, but I can at least show you the alpha.
Since it seems that your alpha masks come as textures themselves, as opposed to having an alpha component to the basecolor, you need to connect the alpha as a separate map, and use the color output (however the color space should be set to non color - I accidentally left it as sRGB in my image, and it works, but this is bad form because that is not always the case).
While you are correct that mixing A diffuse BSDF with a Transparent BSDF, using the alpha as a mix factor, will achieve transparency in some cases, the fact that your alpha is a separate map won't allow this to work.
Instead, try connecting as shown below. Since you are already using the Principled BSDF, it makes sense that you make use of it's alpha channel.
Also note the blend modes I've highlighted on the right. These are necessary to make transparency work (default is opaque). I think alpha-hashed or alpha-blend would work best for you, but try and see for yourself.

Good luck.